1. Field of the Invention
The present invention relates to a standard radio frequency signal generating method and to a standard radio frequency signal generating device, which heterodyne detect CW light which has two different wavelengths, and generate the difference frequency signal.
2. Description of the Related Art
Conventionally, there are two principal standard radio frequency signal generating methods. The first of these standard radio frequency signal generating methods is one which takes advantage of the difference frequency between light beams output from two CW lasers. The second of these standard radio frequency signal generating methods is one in which an optical frequency comb consisting of a plurality of line spectra spaced at a fixed frequency separation is generated, two line spectra are selected from these line spectra, and their difference frequency is taken advantage of.
FIG. 7 is a block diagram showing an example of the structure of a standard radio frequency signal generating device to which the above described first conventional standard radio frequency signal generating method has been applied (refer to, for example, “Exact millimeter wave frequency synthesis by injection locked laser comb line selection” by 0. P. Gough, C. F. C. Silva, and A. J. Seeds, 1999 IEEE International Topical Meeting on Microwave Photonics, Melbourne, Australia, W-4.3, pp. 61-64, 1999). The light beams from two wavelength-tunable lasers 10-1, 10-2 are combined by an optical coupler 11, and are heterodyne detected by a photo-detector (photodiode) 12. It is possible to change the beat frequency as desired by changing the wavelengths of the lasers.
FIG. 8 is a block diagram showing an example of the structure of a standard radio frequency signal generating device to which the above described second conventional standard radio frequency signal generating method has been applied (refer to, for example, “Limit of Optical Frequency Comb Generation due to Material Dispersion” by M. Kourogi, B. Widiyatomoko, Y. Takeuchi, and M. Ohtsu, IEEE Journal of Quantum Electronics, vol. 31, no. 12, pp. 2120-2126, December 1995). As a method for generating an optical frequency comb at a fixed frequency separation, there are a method in which a mode locked pulse laser is employed, or a method in which light of a single frequency is passed through a Fabry-Perot resonator which installs a phase modulator which performs phase modulation at high amplitude. In the spectrum of light output from a fixed frequency separation optical frequency comb generator 13, a plurality of line spectra are generated at separations which are equal to the modulation frequency. Two of these line spectra are selected by a wavelength filtering device 15 and are combined by an optical coupler 11, and the beat signal thereof is heterodyne detected by a photo-detector (photodiode) 12.
With the above described first standard radio frequency signal generating method, two laser light sources are required, and the spectral line width of the beat signal is determined by the line width of the two lasers. Accordingly, if a DFB laser is employed, it is difficult to make the spectral line width of the beat signal be equal to or less than a few tens of kHz. Furthermore, it is extremely difficult to stabilize the oscillation frequency of the laser light sources to the order of kHz. As a result, there is the problem that the frequency of the beat signal fluctuates over time.
Furthermore, with the above described second standard radio frequency signal generating method, it is possible to manage with a single laser light source. However, if a mode locked pulse laser is employed as the optical frequency comb generator, its repetition frequency is limited by the cavity length of the laser. Due to this, there is the problem that it is not possible to vary the beat frequency as desired which is an integral multiple of the repetition frequency. In the same manner, with an optical frequency comb generator which takes advantage of a phase modulator within a Fabry-Perot resonator as well, there is still the problem that it is not possible to vary the beat frequency as desired, since the modulation frequency is limited by the resonator length of the Fabry-Perot resonator.